1. Technical Field
The present invention relates to a stereo camera that measures distance to an object based on parallax of the object.
2. Background Art
Conventionally, a driver support system such as an adaptive cruise control (ACC) has been developed to measure a distance between a driver's vehicle and a vehicle ahead the driver's vehicle and to adjustment a speed the driver's vehicle and maintain the distance to other vehicle. As a technique for measuring a distance to a vehicle ahead of the driver's vehicle, a stereo camera can be used. The stereo camera computes position information of an object by analyzing images captured by two image capturing units and having parallax with respect to the object. For example, U.S. Pat. No. 6,687,003 discloses such a stereo camera. As to the stereo camera disclosed in U.S. Pat. No. 6,687,003, left and right images are converted into spectrum images having different spectrum properties such as blue light image and red light image. One of the converted light images is incident to a front face of a half-mirror and other light image is incident to a rear face of the half-mirror via a mirror. The light image incident to the front face passes through the half-mirror and the light image incident to the rear face of the half-mirror is reflected on the half-mirror so that the two light images are superimposed on the same light path. The superimposed light images enter an image capturing element via a lens, in which the superimposed light images are separated by a color filter disposed for the image capturing element into each light image and received by each light receiving element disposed for the image capturing element. Based on a parallax value of the two light images having different spectrum properties, distance to the object can be computed.
The above described stereo camera can be applied to a system to display three dimensional images to human eyes, and can be also applied to a system to measure distance requiring higher precision.
In the optical system of U.S. Pat. No. 6,687,003, one of left and right images is guided to a half mirror without intervening an optical device such as a mirror, and the other one of left and right images is guided to the half mirror via the mirror. Therefore, a difference occurs between the light path lengths of left and right images. Even if no difference occurs between the light path lengths of left and right images from the object, positional deviation occurs between the left and right images having the parallax.
As to the stereo camera of U.S. Pat. No. 6,687,003, compared to a case that no difference occurs between the light path lengths of left and right images from the object, when difference occurs between the light path lengths of left and right images from the object, positional deviation by parallax caused by reflection at an optical device such as a mirror disposed in the light path becomes great. When the distance measurement computing is conducted, the left and right images may need to be matched with 0.1 pixel precision. If the positional deviation caused by parallax becomes too great, the left and right images cannot be received by adjacent light receiving elements on an image capturing element, and positional error of several pixels occurs to the corresponding pixels. In this case, the left and right images cannot be matched in the pixel matching process conducted before the distance measurement computing. Therefore, computing error of the distance measurement becomes great, with which distance measurement computing cannot be conducted with high precision.